Where am I wrong?
You're calling the Add
method, but expecting it to return something. It doesn't.
If you're trying to create a new dictionary with the same entries and one new one, you probably want:
var headerParameters = new SortedDictionary<string, string>(signatureParameters)
{
"oauth_signature", Uri.EscapeDataString(oauth_signature)
};
The constructor will create a copy of the existing dictionary, then the collection initializer will add the key/value pair just to the new one.